Centrifugation involves the use of mechanical methods to separate liquids and solids by the application of centrifugal force. Separations that proceed slowly under the force of gravity can be speeded enormously through the use of centrifugation equipment to produce centrifugal forces much larger than the force of gravity. Centrifuges are one type of commonly used centrifugation equipment. Conventional laboratory centrifuges are used for testing and research. Conventional industrial centrifuges are used in diverse areas such as sugar refining, oil and gas drilling and production, chemical manufacture, de-watering diesel and jet fuel, and municipal waste treatment.
A force must be applied acting inward toward the center of rotation to make an object move in a circular path. Otherwise the object would continue on a straight path. The object experiences centrifugal acceleration commonly conceptualized as a centrifugal force that acts in an outward direction on the rotating object. The magnitude of the centrifugal force exerted on the object depends on the object's angular velocity and distance from the center of rotation. Centrifugal acceleration is usually compared with gravitational acceleration at the earth's surface. Consequently, centrifugal force is often expressed as a relative force representing multiples of the force of gravity.
Relative centrifugal force can be calculated according to the following equation, ##EQU1## where D is the diameter of the circular path, in meters; n is the rotational speed, in revolutions per minute (rpm's); and V is the peripheral speed of the object around the center of rotation, in meters per second. In a conventional centrifuge, the force varies radially from zero at the rotational axis to a maximum where D equals the inside diameter of the bowl or chamber.
A cyclone is one type of conventional centrifugation equipment that is stationary. Centrifugal force can be generated inside stationary equipment by introducing a high velocity fluid stream tangentially into a cylindrical, conical chamber forming a vortex. Cyclone separators based on this principal remove liquid drops or solid particles from gases. Smaller devices, called liquid cyclones, separate solid particles from liquids. The high velocity required at the inlet of a liquid cyclone can be obtained with standard pumps. Cyclones with diameters ranging from 100 to 300 millimeters make crude separations of large, relatively heavy, solid particles from liquids. The pressure drop across such cyclones can be about 30 to 60 pounds per square inch (psi). More difficult separations may be accomplished by using banks of small devices called hydroclones. Each hydroclone can be approximately 10 millimeters in diameter, can concentrate or remove small particles and can have a pressure drop of approximately 100 psi.
Other conventional stationary centrifugation devices utilize a helical flow path to separate solids and liquids from flowing liquid mixtures. One example of such a system is disclosed in U.S. Pat. No. 4,343,707. The apparatus disclosed in this patent uses a helical conduit to direct water and suspended solids in a helical flowpath, subjecting the water and solids to centrifugal force. This apparatus also uses magnets mounted around the helical conduit to subject the solids suspended in the water to a magnetic force. This magnetic force and the centrifugal force direct the suspended solids radially outward as the water flows through the helical conduit. This apparatus is a low pressure device and requires the magnets to accomplish the desired separation. The water is split into two outlet nozzles such that water with the suspended solids exits the outer nozzle. The remaining water with the suspended solids removed exits the inward nozzle.
An additional centrifugation device using a helical flowpath is disclosed in U.S. Pat. No. 5,004,552. The apparatus disclosed in this patent separates water and crude oil mixtures and other two-phase mixtures having components of different densities. The mixture is pumped through a conduit in the form of a vertical spiral impelling the heavy phase toward the outside of the spiral. A fraction is withdrawn from the outside of the turns of the spiral and is led downwardly to the bottom of a vertical settling tank. Any light phase removed with the heavy phase floats to the top in the settling tank and is reinduced at the top of the spiral.
U.S. Pat. No. 4,678,558 discloses a further apparatus that uses a helical conduit. This apparatus comprises a helical conduit for constraining a fluid flow established along a closed helical tubular course. The helical tubular course has an inlet and outlet respectively situated at opposite ends. A peripheral outlet is disposed at an outer periphery of the conduit between the inlet and outlet ends. The peripheral outlet is used to remove a peripheral fraction concentrated by centrifugal forces in the outer or inner periphery. An unremoved fraction of the flow is allowed to continue along the helical course to the terminal outlet.
Each apparatus disclosed in the above-described patents, as well as other conventional stationary centrifugation equipment, suffer from several problems. Although helpful for separating materials having large density differentials, these conventional devices are unable to accomplish more difficult separations. This inability is caused by a limit as to the multiples of gravity that can be produced. For example, at higher velocities, cyclones experience turbulence that reduces the cyclones' ability to separate. Conventional stationary centrifugation systems are unable to handle fluids having velocities high enough to generate the large centrifugal forces.